Generally, a ball bearing includes an inner race, an outer race, a plurality of balls mounted between the inner race and the outer race so as to be spaced from each other in the circumferential direction, and a retainer retaining the balls. Although retainers made of metal and thus having high rigidity have been widely used in ball bearings for motors, retainers made of synthetic resin, which are quieter during operation and lighter in weight than retainers made of metal, are also widely used in recent years.
A retainer made of synthetic resin is disclosed in Japanese Unexamined Patent Application Publication No. 2006-226448. This retainer includes two annular members made of synthetic resin and facing each other in the axial direction. A plurality of circumferentially spaced apart pockets are defined between the two annular members in which the respective balls are received.
Also, according to how they are guided, retainers are divided into two types. A retainer of the first type is positioned in the radial direction by contact with a bearing race (“bearing race-guided type”). A retainer of the second type is positioned in the radial direction by contact with the balls (“ball-guided type”). The synthetic resin retainer described in Japanese Unexamined Patent Application Publication No. 2006-226448 is of the ball-guided type.
In the above synthetic resin retainer, the inner surface of each pocket is a concave spherical surface except for a grease reservoir provided at each circumferential end of the pocket. When such a retainer was used in a ball bearing for a motor that rotates at a high speed, there was a situation in which abnormal heat generation in the bearing or abnormal wear of the retainer arose.